Aqueous based coating compositions having improved metallic pigment orientation

ABSTRACT

An aqueous based coating composition contains a polymeric film-forming resin and as an additive from about 1 to about 40 percent by weight based on the resin solids of the composition of an oligomeric ester having an acid value of from about 100 to about 500 of the structure: ##STR1## where X is the residue of a polyol containing from 1 to 50 carbon atoms per hydroxyl group after reaction with an acid or an anhydride, R is an organic moiety from the acid or anhydride and A has an average value of about 2 or greater. 
     The coating compositions are particularly advantageous as metallic pigmented coating compositions for automotive basecoat applications because they exhibit excellent resistance to mottling, have good flop and have reduced sensitivity to fluctuations in relative humidity.

BACKGROUND OF THE INVENTION

The present invention relates to waterborne coating compositions and tomethods for the preparation of multi-layered coated articles utilizingsaid compositions and to the resultant coated articles.

In the coating of substrates such as, for example automobiles, where acoating is desired which provides a lustrous appearance, it has beenwell appreciated to provide the substrate with several coating layers inorder to achieve the desired effect. Therefore, typically a pigmentedcoating composition is first applied to the substrate followed by atransparent topcoat. By transparent is meant a clear coating, that isone which does not contain pigmentation or contains only transparentpigments. Such a coating system is commonly referred to as"clear-over-color" or "color plus clear".

Automotive coatings containing metallic pigments such as aluminum flakeare generally utilized to obtain the glossy lustrous appearance which ischaracteristically sought. In order to achieve the preferred appearanceof such metallic coating compositions it is very important that themetallic pigment orient such that it is parallel to the coated surface.The alignment of the pigment in this fashion provides for the mostdesirable appearance, especially with respect to the "flop" of thecoating. By "flop" is meant the visual change in brightness or lightnessof the metallic aluminum flake with a change in viewing angle, that is,a change from 90 to 180 degrees. The greater the visual change fromlight to dark appearance, the better the flop. The flop accentuates thelines and curves of an automobile; therefore, it is very important inachieving the sought after appearance of the coating. It is also veryimportant that the metallic pigment be uniformly oriented across thesurface of the substrate, otherwise blotchy areas of light and darkcolor will be evident. This condition is commonly known as mottling.

Over the past several years the trend in the automotive industry hasbeen to reduce atmospheric pollution caused by the volatile solventswhich are emitted during the painting process. One approach to emissionscontrol has been the use of waterborne coating compositions as thepigmented color coat in the "color plus clear" system.

Waterborne coating compositions, however, are not without attendantdisadvantages. For example, such coatings often have a narrowapplication window in which excellent film properties are obtained. Thatis, it is difficult to obtain smooth films, free of solvent popping overa wide range of relative humidities. In addition, at high humidities,mottling of the film is frequently observed.

It is desirable, therefore, to have waterborne coating compositionswhich have excellent resistance to mottling and popping, have good flop,and also have reduced sensitivity to fluctuations in relative humidity.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided an aqueousbased coating composition containing a polymeric film-forming resin;characterized in that the aqueous based coating composition contains asan additive from about 1 to about 40 percent by weight based on theresin solids of the composition of an oligomeric ester having an acidvalue of from about 100 to about 500, of the structure: ##STR2## where Xis the residue of a polyol containing from 1 to 50 carbon atoms perhydroxyl group, after reaction with an acid or an anhydride, R is anorganic moiety from the acid or anhydride and A has an average value ofabout 2 or greater.

Also in accordance with the present invention there is provided a methodof forming a multilayered coating on a substrate comprising:

(I) applying to the surface of the substrate as a basecoat a pigmentedaqueous based coating composition containing a polymeric film-formingresin; characterized in that the aqueous based coating compositioncontains from about 1 to about 40 percent by weight based on the resinsolids of the composition of an oligomeric ester of the structure:##STR3## where X is the residue of a polyol after reaction with an acidor an anhydride, R is an organic moiety associated with the acid oranhydride and A has an average value of about 2 or greater;

(II) allowing the composition applied in step (I) to at least partiallydry or cure to form a basecoat on the substrate surface;

(III) applying a clear film forming composition over the basecoat ofstep (II).

(IV) allowing the clear composition of step (III) to at least partiallydry or cure to form a transparent topcoat over said basecoat.

Also provided are coated articles prepared in accordance with theaforedescribed method.

DETAILED DESCRIPTION OF THE INVENTION

The waterborne coating compositions of the present invention contain asessential components a polymeric film forming resin dispersed in aqueousmedium and an oligomeric ester.

The oligomeric ester, which can be represented by the followingstructural formula, ##STR4## can be obtained by reaction between apolyol and an acid or an acid anhydride. When an acid anhydride is used,the acid anhydride is reacted under conditions sufficient to ring openthe anhydride forming the half-ester with substantially nopolyesterification occurring. By substantially no polyesterificationoccurring means that the carboxyl groups resulting from the anhydridering opening do not undergo condensation polymerization. By this ismeant that less than 10, usually less than 5 percent by weight ofcondensation polymer is formed.

Alternately, a suitable oligomeric ester can be obtained by reaction ofa dicarboxylic acid with a polyol such as the reaction of adipic acidwith 1-(3-hydroxy-2,2-dimethylpropyl)-3-hydroxy-2,2-dimethylpropionate.Typically the reactants are combined under conditions sufficient toeffect condensation polymerization with the removal of water.

In the above formula, X is the residue of a polyol after reaction withan acid or anhydride, R is an organic moiety associated with the acid oranhydride and A has an average value of about 2 or greater preferably,the polyol contains from 1 to 50 carbon atoms per hydroxyl group.Usually A is an integer of 2 or greater.

It is believed that the molecular weight of the oligomeric ester is notcritical. The preferred esters obtained in the manner described abovehowever, are usually of low molecular weight. The oligomeric estersgenerally have a number average molecular weight of less than 5000,preferably from about 350 to about 1000.

The number average molecular weight is determined by gel permeationchromatography using a polystyrene standard.

In measuring the number average molecular weight using polystyrene asthe standard, a Waters Associates gel permeation chromatograph Model 201was used. Six micro-Styragel columns were used. Each column measured 30centimeters in length and had an inside diameter of 7.8 millimeters. Adifferential refractometer was used as a detector, and the columns werearranged according to their pore size on the order of 10³, 10⁴, 10⁵,10⁶, 500, 100 Angstroms with the 10³ Angstrom column being the first.Tetrahydrofuran was used as a solvent with a flow rate of 2.0milliliters/minute. The quality of the columns is checked by their"theoretical plate number" determined from orthodichlorobenzene. For thepurpose of this application, those columns with theoretical platenumbers than 3000/30 cm were used.

To determine molecular weight by gel permeation chromatography (GPC),the instrument is first calibrated using a polystyrene standard.Polystyrene standards used were purchased from Pressure ChemicalsCompany, Pittsburgh, Pa. The polystyrene standards have dispersities(dispersity=weight average molecular weight/number average molecularweight) ranging from 1.05 to 1.10. The viscosity average molecularweight of the polystyrene standards used were 850,000; 233,000; 474,000;17,400 and 3,600. To obtain a calibration curve, a set of 0.1 percent(10 milligram polystyrene/1.0 ml tetrahydrofuran) polystyrene solutionsin tetrahydrofuran were prepared, and a GPC chromatogram was obtained.The elution volume of each peak corresponding to a Siren molecularweight of the polystyrene standard was measured, and the data wasplotted on a semilogarithmic paper (logarithm scale in the ordinate andlinear scale in the abscissa). A linear least squares plot to log₁₀(molecular weight) versus elution volume in milliliters is used as acalibration curve. The lowest molecular weight of the polystyrenestandard used was 3,600, and the calibration curve beyond that wasextrapolated down to 100. The upper and lower exclusion limits of thisset of columns are 5,000,000 and 100, respectively, in term ofpolystyrene molecular weight. The sample whose molecular weights are tobe determined was prepared as a 1.0 percent tetrahydrofuran solution.After filtration through a 0.5 micron filter, available from MillaporeCorporation, a 0.5 ml sample size was injected into the columns and aGPC chromatogram obtained under the same experimental conditions as thecalibration. From the resulting calibration curve of molecular weightversus retention time, a molecular weight relative to the standard canbe assigned to the retention times of the sample. The height (H) of thecurve at the corresponding retention times is recorded by the computer.From these height-molecular weight (M) combinations, the followingaverages are calculated: Number average molecular weight=ΣH/ΣH/M. Thisis the number reported.

The oligomeric esters of the claimed invention have an acid value whichgenerally ranges from about 100 to about 500, preferably about 150 toabout 300.

The oligomeric esters are present in the claimed coating compositions inan amount ranging from about 1 percent by weight to about 40 percent byweight, preferably about 1 percent to about 25 percent, the percentagesbased on the resin solids of the composition.

The oligomeric ester can be prepared when an acid anhydride and polyolare contacted together usually by mixing the ingredients together in areaction vessel. Preferably, reaction is conducted in the presence of aninert atmosphere such as nitrogen and optionally in the presence of asolvent to dissolve the solid ingredients and/or to lower the viscosityof the reaction mixture. Examples of suitable solvents are high boilingmaterials and include, for example, ketones such as methyl amyl ketone,diisobutyl ketone, methyl isobutyl ketone; as well as other organicsolvents such as dimethyl formamide and N-methyl-pyrrolidone.

For the ring opening reaction and half-ester formation, a1,2-dicarboxylic anhydride can be used. The reaction temperature ispreferably low, that is, no greater than 135° C., preferably less than120° C., and usually within the range of 70°-135° C., preferably90°-120° C.

The time of reaction can vary widely depending principally upon thetemperature of reaction. The reaction time can be as short as 10 minutesto as long as 24 hours.

Hydroxylic solvents such as butyl alcohol, and glycol ethers such asethylene glycol monobutyl ether, diethylene glycol monobutyl ether, andpropylene glycol monopropyl ether can be added after the reaction.Optionally the oligomeric esters can be neutralized with the addition ofamine such as dimethylethanolamine and triethylamine.

The equivalent ratio of anhydride to hydroxy on the polyol is preferablyat least about 0.8:1 (the anhydride being considered monofunctional) toobtain maximum conversion to the desired half-ester, although ratiosless than 0.8:1 can be used.

Among the anhydrides which can be used in the formation of the desiredpolyesters are those which exclusive of the carbon atoms and theanhydride moiety contain from about 2 to 30 carbon atoms. Examplesinclude aliphatic, including cycloaliphatic, olefinic and cycloolefinicanhydrides and aromatic anhydrides. Substituted aliphatic and aromaticanhydrides are also included within the definition of aliphatic andaromatic provided the substituents do not adversely affect thereactivity of the anhydride or the properties of the resultantpolyester. Examples of suitable substituents are chloro, alkyl oralkoxy. Examples of anhydrides include gluteric anhydride, succinicanhydride, methylsuccinic anhydride, dodecenyl succinic anhydride,octadecenylsuccinic anhydride, phthalic anhydride, tetrahydrophthalicanhydride, methyltetrahydrophthalic anhydride, alkyl substitutedphthalic anhydride, hexahydrophthalic anhydride, alkyl hexahydrophthalicanhydrides such as methylhexahydrophthalic anhydride,tetrachlorophthalic anhydride, endomethylene tetrahydrophthalicanhydride, chlorendic anhydride, itaconic anhydride, citraconicanhydride and maleic anhydride. It should be understood that mixtures ofanhydrides can be used. Preferably methylhexahydrophthalic anhydride ora mixture of methylhexahydrophthalic anhydride and hexahydrophthalicanhydride are utilized.

Among the polyols which can be used are those which contain about 2 to20 carbon atoms. Preferred are diols, triols and mixtures thereof.Examples include polyols containing from 2 to 10 carbon atoms. Examplesinclude aliphatic polyols such as ethylene glycol, 1,2-propanediol,1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, glycerol, 1,2,3-butanetriol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, dipropyleneglycol, 1,4-cyclohexanedimethanol, trimethylolpropane, 2,2,4-trimethylpentane-1,3-diol, pentaerythritol a tetrol,1-(3-hydroxy-2,2-dimethylpropyl)-3-hydroxy-2,2-dimethylpropionate.Aromatic polyols such as bisphenol A and bis(hydroxylmethyl) xylene canalso be used. Preferably 1,4-cyclohexane dimethanol and1-(3-hydroxy-2,2-dimethylpropyl)-3-hydroxy-2,2-dimethyl propionate areutilized.

Alternately a suitable oligomeric ester can be obtained by the reactionof a dicarboxylic acid with a polyol utilizing esterification bycondensation, eliminating water which is removed by distillation. Thereaction temperature is preferably in the range of about 120° C. toabout 230° C. Among the dicarboxylic acids that can be used arealiphatic, cycloaliphatic and aromatic diacids. Examples of dicarboxylicacid that can be used include adipic acid, azelaic acid, sebacic acid,isophthalic acid, terephthalic acid, cyclohexanedicarboxylic acid anddodecanedicarboxylic acid.

Preferred oligomeric esters according to the present invention are thereaction products of1-(3-hydroxy-2,2-dimethylpropyl)-3-hydroxy-2,2-dimethylpropionate withmethylhexahydrophthalic anhydride, 1,4-cyclohexane dimethanol with amixture of methylhexahydrophthalic anhydride and hexahydrophthalicanhydride, and1-(3-hydroxy-2,2-dimethylpropyl)-3-hydroxy-2,2-dimethylpropionate withadipic acid.

The polymeric film-forming resin which is dispersed in the aqueousmedium is preferably an aqueous dispersion of polymeric microparticles.Preferably the microparticles are crosslinked. A wide range ofcrosslinked polymeric microparticle dispersions are suitable for useherein including those described in U.S. Pat. No. 4,403,003 andreferences cited therein. In a preferred embodiment, the microparticlescontain greater than 30 percent by weight based on the microparticles ofa substantially hydrophobic condensation polymer having a molecularweight of greater than 300. The substantially hydrophobic polymer isessentially free of repeating acrylic or vinyl units in the backbone.Preferably the microparticles contain greater than 40 percent by weightof the substantially hydrophobic polymer, more preferably greater than50 percent. By substantially hydrophobic is meant that upon mixing asample of polymer with an organic component and water, a majority of thepolymer is in the organic phase and a separate aqueous phase isobserved. Examples of suitable condensation polymers include polyesters,polyurethanes, polyethers and alkyds which are discussed in detailbelow.

It should be understood that the substantially hydrophobic polymerhaving a molecular weight greater than 300 is adapted to be chemicallybound into the cured coating composition. That is, the polymer isreactive in the sense that it contains functional groups such ashydroxyl groups which are capable of coreacting, for example, with acrosslinking agent such as a melamine formaldehyde resin which may bepresent in the coating composition or alternatively with other filmforming resins which also may be utilized. Preferably, the polymer has amolecular weight greater than 500, more preferably greater than 800.Typically the molecular weight ranges from about 300 to about 10,000,more usually from about 300 to about 2,000. By "essentially free ofrepeating acrylic or vinyl units" is meant that the polymer is notprepared from typical free radically polymerizable monomers such asacrylates, styrene and the like.

As was mentioned above, the polyester, polyurethane, alkyd and polyetherresins are examples of suitable substantially hydrophobic polymers. Thepolyester resins contain essentially no oil or fatty acid modification.That is, while alkyd resins are in the broadest sense polyester typeresins, they are oil-modified and thus not generally referred to aspolyester resins. The polyesters are of two kinds. One type are theunsaturated polyesters derived from unsaturated polyfunctional acids andpolyhydric alcohols. Maleic acid and fumaric acid are the usualunsaturated acid components although methacrylic acid unsaturatedalcohols such as trimethylolpropane mono- or diallyl esters can also beused. Commonly used polyhydric alcohols are 1,4-butanediol,1,6-hexanediol, neopentyl glycol, ethylene glycol, propylene glycol,diethylene glycol, dipropylene glycol, butylene glycol, glycerol,trimethylolpropane, pentaerythritol and sorbitol. Often times asaturated acid will be included in the reaction to provide desirableproperties. Examples of saturated acids include phthalic acid,isophthalic acid, adipic acid, azeleic acid, sebacic acid, and theanhydrides thereof. The saturated polyesters are derived from saturatedor aromatic polyfunctional acids, preferably dicarboxylic acids, andmixtures of polyhydric alcohols having an average hydroxyl functionalityof at least 2. Other components of polyesters can include hydroxy acidand lactones such as ricinoleic acids, 12-hydroxystearic acid,caprolactone, butyrolactone and dimethylolopropionlc acid.

The alkyds are polyesters of polyhydroxyl alcohols and polycarboxylicacids chemically combined with various drying, semi-drying andnon-drying oils in different proportions. Thus, for example, the alkydresins are made from polycarboxylic acids such as phthalic acid, maleicacid, fumaric acid, isophthalic acid, succinic acid, adipic acid,azeleic acid, sebacic acid as well as from anhydrides of such acids,where they exist. The polyhydric alcohols which can be reacted with thepolycarboxylic acid include 1,4-butanediol, 1,6-hexanediol, neopentylglycol, ethylene glycol, diethylene glycol and 2,3-butylene glycol,glycerol, trimethylolpropane, trimethylolpropane, pentaerythritol,sorbitol and mannitol.

The alkyd resins are produced by reacting the polycarboxylic acid andthe polyhydric alcohols together with a drying, semi-drying ornon-drying oil in proportions depending upon the properties desired.

The oils are coupled into the resin molecule by esterification duringthe manufacturing and become an integral part of the polymer. The oil isfully saturated or predominately unsaturated. When cast into films, thefully saturated oils tend to crosslink and dry rapidly with oxidation togive more tough and solvent resistant films. Suitable oils includecoconut oil, fish oil, linseed oil, tung oil, castor oil, cottonseedoil, safflower oil, soybean oil, and tall oil. Various proportions ofthe polycarboxylic acid, polyhydric alcohol and oil are used to obtainalkyd resins of various properties as is well known in the art.

Examples of polyether polyols are polyalkylene ether polyols whichinclude those having the following structural formula: ##STR5## wherethe substituent R is hydrogen or lower alkyl containing from 1 to 5carbon atoms including mixed substituents, and n is typically from 2 to6 and m is from 10 to 100 or even higher. Included arepoly(oxytetraethylene) glycols, poly(oxy-1,2-propylene) glycols andpoly(oxy-1,2-butylene) glycols.

Also useful are polyether polyols formed from oxyalkylation of variouspolyols, for example, glycols such as trimethylolpropane,pentaerythritol and the like. Polyols of higher functionality which canbe utilized as indicated can be made, for instance, by oxyalkylation ofcompounds as sorbitol or sucrose. One commonly utilized oxyalkylationmethod is by reacting a polyol with an alkylene oxide, for example,ethylene or propylene oxide, in the presence of an acidic or basiccatalyst.

With polyether polyols, it is preferred that the carbon to oxygen weightratio be high for better hydrophobic properties. This it is preferredthat the carbon to oxygen ratio be greater than 3/1 and more preferablygreater than 4/1.

The polyurethane resins can be prepared by reacting a polyol with apolyisocyanate. The reaction can be performed with a minor amount oforganic polyisocyanate (OH/NCO equivalent ratio greater than 1:1) sothat terminal hydroxyl groups are present or alternatively the OH/NCOequivalent ratio can be less than 1:1 thus producing terminal isocyanategroups. Preferably the polyurethane resins have terminal hydroxylgroups.

The organic polyisocyanate can be an aliphatic polyisocyanate, includinga cycloaliphatic polyisocyanate or an aromatic polyisocyanate. Usefulaliphatic polyisocyanates include aliphatic diisocyanates such asethylene diisocyanate, 1,2-diisocyanatopropane, 1,3-diisocyanatopropane,1,6-diisocyanatohexane, 1,4-butylene diisocyanate, lysine diisocyanate,1,4-methylene bis(cyclohexyl isocyanate) and isophorone diisocyanate.Useful aromatic diisocyanates and aliphatic diisocyanates includevarious isomers of toluenes diisocyanate, meta-xylenediisocyanate andparaxylene-diisocyanate, also 4-chloro-1,3-phenylene diisocyanate,1,5-tetrahydronaphthalene diisocyanate, 4,4'-dibenzyl diisocyanate and1,2,4-benzene triisocyanate can be used. In addition the various isomersor alpha, alpha, alpha', alpha'-tetramethylxylene diisocyanate can beused. Also useful as the polyisocyanate are isocyanurates such asDESMODUR 3300 from Miles, Inc. and biurets of isocyanates such asDESMODUR NIOO from Miles, Inc.

The polyol can be polymeric such as the polyester polyols, polyetherpolyols, polyurethane polyols, etc., or it can be simple diol or triolsuch as ethylene glycol, propylene glycol, butylene glycol, glycerol,trimethylolpropane or hexanetriol. Mixtures can also be utilized.

The balance of the microparticle comprises a polymer of a vinyl monomeror mixture of vinyl monomers. These monomers are referred to herein aspolymerizable species. Examples of suitable materials include acrylicmonomers including alkyl esters of acrylic and methacrylic acid, such asmethyl acrylate, methyl methacrylate, butyl acrylate, butylmethacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate,2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, hydroxypropylmethacrylate, styrene, acrylamide, acrylonitrile, alkyl esters of maleicand fumaric acid, vinyl and vinylidene halides, acrylic acid, ethyleneglycol dimethacrylate, isobornyl methacrylate, vinyl acetate, vinylethers, allyl ethers, lauryl methacrylate, and N-butoxymethylacrylamide. Preferably the vinyl monomer used to prepare the polymerwhich comprises the balance of the microparticle is selected fromacrylic monomers. The polymer having a molecular weight greater than 300is also substantially insoluble in the aqueous medium and is alsocapable of being dissolved in the monomer mixture which is utilized toprepare the polymer which comprises the balance of the microparticle.

The dispersion of polymeric microparticles in an aqueous medium ispreferably prepared by a high stress technique which is described morefully below. First, the vinyl monomer or mixture of vinyl monomersutilized to prepare the polymer which comprises the balance of themicroparticle is thoroughly mixed with the aqueous medium and thesubstantially hydrophobic polymer having a molecular weight greater than300. For the present application, the vinyl monomer or mixture of vinylmonomers together with the substantially hydrophobic polymer is referredto as the organic component. The organic components generally alsocomprise other organic species and preferably is substantially free oforganic solvent. That is, no more than 20 percent of organic solvent ispresent. The mixture is then subjected to stress in order to particulateit into microparticles which are uniformly of a fine particle size. Themixture is submitted to stress sufficient to result in a dispersion suchthat after polymerization less than 20 percent of the polymermicroparticles have a mean diameter greater than 5 microns.

The aqueous medium provides the continuous phase of dispersion in whichthe microparticles are suspended. The aqueous medium is generallyexclusively water. However, for some polymer systems, it may bedesirable to also include a minor amount of inert organic solvent whichcan assist in lowering the viscosity of the polymer to be dispersed. Forexample, if the organic phase has a Brookfield viscosity greater than1000 centipoise at 25° C. or a W Gardner Holdt viscosity the use of somesolvent may be preferred. For some applications of the aqueousmicroparticle dispersion, for example, in its present use as a resinousbinder for coating compositions, it may be desirable to have acoalescing solvent for the coating composition. One can convenientlyinclude this coalescing solvent during the synthesis of the latex aspart of the organic component. Examples of suitable water insolublesolvents which can be incorporated in the organic component are benzylalcohol, xylene, methyl isobutyl ketone, mineral spirits, butanol, butylacetate, tributyl phosphate and dibutyl phthalate.

As was mentioned above, the mixture is subjected to the appropriatestress by use of a MICROFLUIDIZER® emulsifier which is available fromMicrofluidics Corporation in Newton, Mass. The MICROFLUIDIZER® highpressure impingement emulsifier is patented in U.S. Pat. No. 4,533,254.The device consists of a high pressure (up to 20,000 psi) pump and aninteraction chamber where the emulsification takes place. The pumpforces the mixture of reactants in aqueous medium into the chamber whereit is split into at least two streams which pass at very high velocitythrough at least two slits and collide resulting in the particulates ofthe mixture into small particles. Generally, the reaction mixture passedthrough the emulsifier once at a pressure between 5,000 and 15,000 psi.Multiple passes can result in smaller average particle size and anarrower range for the particle size distribution. When using theaforesaid MICROFLUIDIZER® emulsifier, stress is applied by liquid-liquidimpingement as has been described. However, it should be understood thatif desired, other modes of applying stress to the pre-emulsificationmixture can be utilized so long as sufficient stress is applied toachieve the requisite particle size distribution, that is, such thatafter polymerization less than 20 percent of the polymer microparticleshave a mean diameter greater than 5 microns. For example, onealternative manner of applying stress would be the use of ultrasonicenergy.

Stress is described as force per unit area. Although the precisemechanism by which the MICROFLUIDIZER® emulsifier stresses thepre-emulsification mixture to particulate it is not thoroughlyunderstood. It is theorized that stress is exerted in more than onemanner. It is believed that one manner in which stress is exerted is byshear. Shear means that the force is such that one layer or plane movesparallel to an adjacent, parallel plane. Stress can also be exerted fromall sides as a bulk, compression stress. In this instance stress couldbe exerted without any shear. A further manner of producing intensestress is by cavitation. Cavitation occurs when the pressure within aliquid is reduced enough to cause vaporization. The formation andcollapse of the vapor bubbles occurs violently over a short time periodand produces intense stress. Although not intending to be bound bytheory, it is believed that both shear and cavitation contribute toproducing the stress which particulates the pre-emulsification mixture.

Once the mixture has been particulated into microparticles, thepolymerizable species within each particle are polymerized underconditions sufficient to produce polymer microparticles which are stablydispersed in the aqueous medium. It should be understood that one of therequisite conditions sufficient to achieve the stably dispersedmicroparticles is the presence in the reaction mixture of a surfactantwhich is also termed a dispersant. The surfactant is preferably presentwhen the organic component referred to above is mixed into the aqueousmedium, prior to particulation. Alternatively, the surfactant can beintroduced into the medium at a point just after the particulationwithin the MICROFLUIDIZER® emulsifier. The surfactant, however, can bean important part of the particle forming process and is often necessaryto achieve the requisite dispersion stability. The surfactant can be amaterial whose role is to prevent the emulsified particles fromagglomerating to form larger particles.

The same surfactants or dispersants which can be utilized duringconventional emulsion polymerization are also suitable for this highstress technique. Examples of suitable surfactants include thedimethylethanolamine salt of dodecylbenzenesulfonic acid, sodiumdioctylsulfosuccinate, ethoxylated nonylphenol and sodium dodecylbenzene sulfonate. Other materials well known to those skilled in theart are also suitable herein. Generally, both ionic and nonionicsurfactants are used together and the amount of surfactant ranges fromabout 1 percent to about 10 percent, preferably from about 2 percent toabout 4 percent, the percentage based on the total solids. Oneparticularly preferred surfactant for the preparation of aminoplastcurable dispersions is the dimethylethanolamine salt ofdodecylbenzenesulfonic acid.

In order to conduct the free radical polymerization of the polymerizablespecies a free radical initiator is also required. Both water solubleand oil soluble initiators can be used. Since the addition of certaininitiators, such as redox initiators, can result in a strong exothermicreaction, it is generally desirable to add the initiator to the otheringredients immediately before the reaction is to be conducted. Examplesof water soluble initiators include ammonium peroxydisulfate, potassiumperoxydisulfate and hydrogen peroxide. Examples of oil solubleinitiators include t-butyl perbenzoate and2,2'-azobis(isobutyronitrile). Preferably redox initiators such asammonium peroxydisulfate/sodium metabisulfite ort-butylhydroperoxide/isoascorbic acid are utilized herein.

It should be understood that in some instances it may be desirable forsome of the reactant species to be added after particulation of theremaining reactants and the aqueous medium. For example, water solubleacrylic monomers such as hydroxypropyl methacrylate.

The particulated mixture is then subjected to conditions sufficient toinduce polymerization of the polymerizable species, within themicroparticles. The particular conditions will vary depending upon theactual materials being polymerized. The length of time required tocomplete polymerization typically varies from about 10 minutes to about6 hours.

The progress of the polymerization reaction can be followed bytechniques conventionally known to those skilled in the art of polymerchemistry. For example, heat generation, monomer concentration andpercent of total solids are all methods of monitoring the progress ofthe polymerization.

The aqueous microparticle dispersions can be prepared by a batch processor a continuous process. In one batch process the unreactedmicrodispersion is fed over a period of about 1 to 4 hours into a heatedreactor initially charged with water. The initiator can be fed insimultaneously, it can be part of the microdispersion or it can becharged to the reactor before feeding in the microdispersion. Theoptimum temperature depends upon the specific initiator being used. Thelength of time typically ranges from about 2 hours to about 6 hours.

In an alternative batch process, a reactor vessel is charged with theentire amount of microdispersion to be polymerized. Polymerizationcommences when an appropriate initiator such as a radox initiator isadded. An appropriate initial temperature is chosen such that the heatof polymerization does not increase the batch temperature beyond theboiling point of the ingredients. Thus for large scale production, it ispreferred that the microdispersion have sufficient heat capacity toabsorb the total amount of heat being generated.

In a continuous process the pre-emulsion or mixture of raw materials ispassed through the homogenizer to make a microdispersion which isimmediately passed through a heated tube, e.g., stainless steel, or aheat exchanger in which polymerization takes place. The initiator isadded to the microdispersion just before it enters the tubing.

It is preferred to use radox type initiators in the continuous processsince other initiators can produce gases such as nitrogen or carbondioxide which can cause the latex to spurt out of the reaction tubingprematurely. The temperature of reaction can range from about 25° C. toabout 80° C., preferably about 35° C. to about 45° C. The residence timetypically ranges from about 5 minutes to about 30 minutes.

The tubing in which the reaction occurs is not required to heat themicrodispersion but rather to remove the heat being generated. Once theinitiator has been added, the reaction begins spontaneously after ashort induction period and the reaction exotherm resulting from thepolymerization will rapidly raise the temperature.

If there is still free monomer remaining after all of the initiator isconsumed, an additional amount of initiator can be added to scavenge theremaining monomer.

Once the polymerization is complete, the resultant product is a stabledispersion of polymer microparticles in an aqueous medium, wherein boththe polymer formed from the polymerizable species and the substantiallyhydrophobic polymer of greater than 300 molecular weight are containedwithin each microparticle. The aqueous medium, therefore, issubstantially free of water soluble polymer. The resultant polymermicroparticles are of course insoluble in the aqueous medium. In sayingthat the aqueous medium is substantially free of water soluble polymer,it is intended that the term "substantially free" means that the aqueousmedium contains no more than 30 percent by weight of dissolved polymer,preferably no more than 15 percent.

By "stably dispersed" is meant that the polymer microparticles do notsettle upon standing and do not coagulate or flocculate on standing.Typically, when diluted to 50 percent total solids the microparticledispersions do not settle even when aged for one month at roomtemperature.

As was stated above, a very important aspect of the polymermicroparticle dispersions is that the particle size is uniformly small,i.e., after polymerization less than 20 percent of the polymermicroparticles have a mean diameter which is greater than 5 microns,more preferably greater than 1 micron. Generally, the microparticleshave a mean diameter from about 0.01 microns to about 10 microns.Preferably the mean diameter of the particles after polymerizationranges from about 0.05 microns to about 0.5 microns. The particle sizecan be measured with a particle size analyzer such as the Coulter N4instrument commercially available from Coulter. The instrument comeswith detailed instructions for making the particle size measurement.However, briefly, a sample of the aqueous dispersion is diluted withwater until the sample concentration falls within specified limitsrequired by the instrument. The measurement time is 10 minutes.

The microparticle dispersions are high solids materials of lowviscosity. Dispersions can be prepared directly with a total solidscontent of from about 45 percent to about 60 percent. They can also beprepared at a lower solids lever of about 30 to about 40 percent totalsolids and concentrated to a higher level of solids of about 55 to about65 percent by stripping. The molecular weight of the polymer andviscosity of the aqueous dispersions are independent of each other. Theweight average molecular weight can range from few hundred to greaterthan 100,000. The Brookfield viscosity can also vary widely from about0.01 poise to about 100 poise, depending on the solids and composition,preferably from about 0.2 to about 5 poise when measured at 25° C. usingan appropriate spindle at 50 RPM.

The microparticle dispersion can be either crosslinked or uncrosslinked.When uncrosslinked the polymer within the microparticle can be eitherlinear or branched.

Additionally the polymeric film-forming resin of the claimed waterbornecomposition can be a water soluble polymer or copolymer well known tothose skilled in the art.

The oligomeric ester can be incorporated into the claimed coatingcompositions by addition with other constituents of the coatingcompositions (i.e., film forming resin, crosslinking agents, pigments,etc.). The addition occurs with the coating composition under agitation.The pH is then adjusted to the normal operating range of about 8.0 to9.0 by the addition of an amine such as dimethylethanolamine ortriethylamine.

Alternately with the preferred polymeric microparticles described indetail above, although not preferred, the oligomeric ester can beincorporated during the preparation of the polymeric microparticles.Specifically, the oligomeric ester can be cold blended with thepolymeric microparticles and then returned to the MICROFLUIDIZER®. Inaddition the oligomeric ester can be blended with the vinyl monomerdescribed in detail above, as a first step in the preparation ofpolymeric microparticles described above in detail.

The coating compositions of the claimed invention, in a preferredembodiment, additionally comprise a crosslinking agent which is adaptedto cure the polymeric microparticles, such as an aminoplast crosslinker.

Aminoplast resins are based on the addition products of formaldehyde,with an amino- or amido-group carrying substance. Condensation productsobtained from the reaction of alcohols and formaldehyde with melamine,urea or benzoguanamine are most common and preferred herein. However,condensation products of other amines and amides can also be employed,for example, aldehyde condensates of triazines, diazines, triazoles,guanadines, guanamines and alkyl- and aryl-substituted melamines. Someexamples of such compounds are N,N'-dimethyl urea, benzourea,dicyandiamide, formaguanamine, acetoguanamine, glycoluril, anneline,2-chloro-4,6-diamino-1,3,5-triazine, and the like.

While the aldehyde employed is most often formaldehyde, other similarcondensation products can be made from other aldehydes, such asacetaldehyde, crotonaldehyde, acrolein, benzaldehyde, furfuryl, glyoxaland the like.

The aminoplast resins contain methylol or similar alkylol groups, and inmost instances, at least a portion of these alkylol groups areetherified by a reaction with an alcohol to provide organic solventsoluble resins. Any monohydric alcohol can be employed for this purpose,including such alcohols as methanol, ethanol, propanol, butanol,pentanol, hexanol, heptanol and others, as well as benzyl alcohol andother aromatic alcohols, cyclic alcohols such as cyclohexanol,monoethers of glycols such as Cellosolves and Carbitols, and halogensubstituted or other substituted alcohols, such as 3-chloropropanol orbutoxyethanol. The preferred aminoplast resins are substantiallyalkylated with methanol or butanol.

The claimed coating compositions can contain, in addition to thecomponents described above, a variety of other optional materials. Aswas mentioned above, if desired, other resinous materials can beutilized in conjunction with the dispersion of polymeric microparticlesso long as the resultant coating composition is not detrimentallyaffected in terms of physical performance and properties. In addition,material such as rheology control agents, ultraviolet light stabilizers,catalysts, fillers and the like can be present.

As was mentioned above, the waterborne coating compositions of thepresent invention are particularly suitable as basecoating compositionsin automotive color plus clear applications. For this applicationpigment is one of the principal ingredients. The pigments which can beutilized are of various types, depending upon whether a metallic pigmentis desired. When a metallic coating is desired preferably aluminum flakeis utilized. A variety of grades of aluminum flake are available such asSilberline Sparkle Silver 5000 AR, Toyo 8260 and Obron OBT 8167 STAPA M.Also chrome treated aluminum flake such as Hydrolux 400 and Ekkert 47700can be used. Other metallic pigments include bronze flakes, coated mica,nickel flakes, tin flakes, silver flakes, copper flakes, or combinationof these. Other examples of suitable pigments include mica, iron oxides,lead oxides, carbon black, titanium dioxide, talc, as well as a varietyof color pigments. The specific pigment to binder ratio can vary widelyso long as it provides the requisite hiding at the desired filmthickness and application solids.

As automotive waterborne basecoat compositions, the compositions of thepresent invention are very advantageous, particularly in basecoatscontaining metallic pigments. The coating compositions are particularlyresistant to mottling. (By "mottling" is meant the irregular orientationof metallic pigments in the deposited film causing blotchy areas oflight and dark color.) The compositions have good leveling and flowcharacteristics and exhibit an excellent automotive quality finish whichis demonstrated by the excellent flop of the coating. The compositionsalso have low volatile organic content. Generally the volatile organiccontent is less than 3.5 pounds per gallon. In addition, the claimedcoating compositions used as basecoats are very versatile and can beutilized with a variety of clear coating compositions as topcoatsincluding solvent borne clear coats, waterborne clear coats and powderclear coats.

The claimed coating compositions can be applied by conventional meansincluding brushing, dipping, flow coating, spraying and the like, butthey are most often applied by spraying. Conventionally known spraytechniques and equipment for air spraying and electrostatic spraying andeither manual or automatic methods can be used.

During application of the basecoat composition to the substrate, a filmof the basecoat is formed on the substrate. Typically, the basecoatthickness will be about 0.01 to 5 mils, preferably 0.1 to 2 mils inthickness.

After application to the substrate of the base coat composition, a filmis formed on the surface of the substrate. This is achieved by drivingsolvent, i.e., organic solvent and water, out of the base coat film byheating or simply by an air-drying period. Preferably, the heating stepwill be for a period to insure that the top coat composition can beapplied to the base coat without the former dissolving the base coatcomposition, i.e., "striking in". Suitable drying conditions will dependon the particular base coat composition, on the ambient humidity withcertain waterbased compositions, but in general a drying time of fromabout 1 to 5 minutes at a temperature of about 100° to 250° F. (39° to121° C.) will be adequate to insure that mixing of the two coats isminimized. At the same time, the base coat film is adequately wetted bythe top coat composition so that satisfactory intercoat adhesion isobtained. Also, more than one base and multiple top coats may be appliedto develop the optimum appearance. Usually between coats, the previouslyapplied base coat or top coat is flashed, that is, exposed to ambientconditions for about 1 to 20 minutes. The clear top coat composition canbe applied to the basecoated substrate by any of the conventionalcoating techniques described above in connection with the basecoat, butit is preferred that spray applications be used since this gives thebest gloss.

After application of the top coat composition to the base coat, thecoated substrate is heated to cure the coating layers. In the curingoperation, the film-forming material of the top coat and/or of the basecoat is crosslinked with the aid of any crosslinking agents present. Theheating or curing operation is usually carried out at a temperature inthe range of from 160°-350° F.(71°-177° C.) but if needed lower orhigher temperatures may be used depending upon whether it is sufficientto activate any necessary crosslinking mechanisms.

It should be understood that for the purposes of the present inventionthe term "curing" also includes drying without any externally addedcrosslinking agent.

The thickness of the topcoat is usually from about 0.5 to 5, preferably1.2 to 3 mils.

The invention will be further described by reference to the followingexamples. Unless otherwise indicated, all parts are by weight.

EXAMPLE I

A polyurethane acrylate was prepared from the following ingredients:

    ______________________________________                                        Amount (g)                                                                             Material                                                             ______________________________________                                        1000     poly(neopentyl glycol adipate) having number                                  average molecular weight of 1000, commercially                                available as FORMREZ 55-112 (Witco)                                  116      hydroxyethyl acrylate (HEA)                                          1.4      dibutyltin dilaurate                                                 1.4      butylated hydroxytoluene                                             244      tetramethyl xylene diisocyanate (TMXDI)                              340      butyl acrylate (BA)                                                  ______________________________________                                    

The first four ingredients were stirred in a flask as the TMXDI wasadded over a one hour period at a temperature of 70°-76° C. 90 g of thebutyl acrylate was used to rinse the addition funnel containing theTMXDI and the temperature of the mixture was then held at 70° C. for anadditional 2 hours as all the isocyanate reacted. The remainder of thebutyl acrylate was added to produce an 80% solution with a Gardner-Holdtviscosity of X, an acid value of 0.8, and a hydroxyl value of 29.

A pre-emulsion was prepared by stirring together the followingingredients:

    ______________________________________                                        Amount (g)                                                                             Material                                                             ______________________________________                                        750.0    polyurethane prepolymer from above                                   110.0    methyl methacrylate (MMA)                                            90.0     butyl acrylate (BA)                                                  30.0     ethylene glycol dimethacrylate (EGDMA)                               20.0     acrylic acid (AA)                                                    33.3     ALIPAL Co-436 (60% solution of ammonium                                       nonylphenol tetra-ethyleneoxy sulfate,                                        available from the GAF Corporation)                                  29.9     PGNP-15 (polyglycidyl nonylphenol,                                            available from the Dixie Chemical Co.)                               13.3     AEROSOL OT-75 (75% solution of sodium dioctyl                                 sulfosuccinate available from American Cyanamid)                     5.9      70% solution of dodecylbenzenesulfonic acid                          1.4      N,N-dimethyl ethanolamine (DMEA)                                     2.0      1% aqueous solution of ferrous ammonium sulfate                               (FAS)                                                                0.12     FOAMKILL 649                                                         700.0    water                                                                ______________________________________                                    

The pre-emulsion was passed twice through a M110 Microfluidizer® at 8000psi to produce a bluish-white emulsion. The emulsion was transferred toa fourneck round bottom flask equipped with a thermometer, mechanicalstirrer, condenser and a nitrogen line. The polymerization was initiatedby adding first a mixture of 1.5 g isoascorbic acid dissolved in 350 gwater followed by a solution of 1.5 g 35% hydrogen peroxide in 250 gwater added over a fifteen minute period. The temperature of theemulsion went from 24° C. to 59° C. The pH of the latex was adjustedfrom 3.6 to 8.2 by the addition of 41.0 g of a 33% solution of aqueousDMEA. Finally, 1.9 g of Proxel GXL (biocide available from ICI)dissolved in 13 g of water was added. The total solids of the latex was42.9% and the Brookfield viscosity (50 rpm, #1 spindle) was 31 cps.

EXAMPLE A

A polyacid half-ester of1-(3-hydroxy-2,2-dimethylpropyl)-3-hydroxy-2,2-dimethylpropionate (EsterDiol 204) and methylhexahydrophthalic anhydride (ED 204/MHHPA) wasprepared from the following mixture of ingredients:

    ______________________________________                                                              Parts by Weight                                         Ingredients           (in grams)                                              ______________________________________                                        Ester Diol 204        2550.0                                                  Methylhexahydrophthalic anhydride                                                                   4116.0                                                  Methyl isobutyl ketone                                                                              1466.5                                                  Ethanol                666.6                                                  ______________________________________                                    

The Ester Diol 204 and 1466.5 grams of the methyl isobutyl ketone werecharged to a reaction vessel and heated under a nitrogen atmosphere to115° C. The methylhexahydrophthalic anhydride was added over a 90-minuteperiod. The reaction mixture was then held at to 115° C. for four hours.The reaction mixture was then cooled to 100° C. followed by the additionof ethanol (to react with residual anhydride) and heating the reactionmixture to reflux and holding for two hours. The reaction mixture wasstripped to a pot temperature of 125° C. The reaction mixture was thenadjusted to 80 percent solids with methyl isobutyl ketone.

EXAMPLE B

A polyacid half-ester of di-trimethylolpropane andmethylhexahydrophthalic anhydride (di-TMP/MHHPA) was prepared from thefollowing mixture of ingredients:

    ______________________________________                                        Ingredients           Weight in grams                                         ______________________________________                                        Di-Trimethylolpropane 1584.8                                                  Methylhexahydrophthalic anhydride                                                                   4120.7                                                  Methyl isobutyl ketone                                                                               570.5                                                  n-Propyl alcohol      2114.4                                                  ______________________________________                                    

The di-trimethylolpropane and 540.5 grams of methyl isobutyl ketone werecharged to a reaction vessel and heated under a nitrogen atmosphere to115° C. The methylhexahydrophthalic anhydride was added over a period ofabout 2 hours at 115° C. The remainder of the methyl isobutyl ketone wasadded as a rinse. The reaction was held at 115° C. for 4 hours. Thereaction mixture was then cooled to 100° C., and the n-propyl alcoholwas added. The reaction mixture was then heated to 105° C. and held for2 hours and then cooled to room temperature. The reaction mixture had asolids content of 72.3 percent and an acid value of 163.

EXAMPLE C

A polyacid half-ester (TMP/MHHPA) of trimethylolpropane andmethylhexahydrophthalic anhydride was prepared from the followingmixture of ingredients:

    ______________________________________                                        Ingredients           Weight in grams                                         ______________________________________                                        Trimethylolpropane     588.1                                                  Methylhexahydrophthalic anhydride                                                                   2208.5                                                  Methyl isobutyl ketone                                                                              1198.4                                                  Ethyl alcohol          279.2                                                  ______________________________________                                    

The trimethylolpropane and 1065.4 grams of methyl isobutyl ketone werecharged to a reaction vessel and heated under a nitrogen atmosphere to115° C. The methylhexahydrophthalic anhydride was added over a period ofabout 2 hours at 115° C. The remainder of the methyl isobutyl ketone wasadded to the rinse. The reaction was held at 115° C. for 4 hours. Thereaction mixture was then cooled to 100° C., and the ethyl alcohol wasadded. The reaction mixture was then heated to 105° C. and held for 2hours and then stripped to a reaction temperature of 125° C. to removethe ethyl alcohol. A total of 495 grams of solvent was removed. Thereaction mixture was cooled to room temperature and 215 grams of methylisobutyl ketone was added to the reaction mixture to adjust solids toabout 70 percent. The reaction mixture had a solids content of 69.9percent and an acid value of 190.

EXAMPLE D

A polyacid half-ester of neopentyl glycol and methylhexahydrophthalicanhydride (NPG/MHHPA) was prepared from the following mixture ofingredients:

    ______________________________________                                        Ingredients           Weight in grams                                         ______________________________________                                        Neopentyl glycol      1300.0                                                  Methylhexahydrophthalic anhydride                                                                   4116.0                                                  Methyl isobutyl ketone                                                                              2321.1                                                  Ethyl alcohol          541.6                                                  ______________________________________                                    

The neopentyl glycol and 2121.1 grams of methyl isobutyl ketone werecharged to a reaction vessel and heated under a nitrogen atmosphere to115° C. The methylhexahydrophthalic anhydride was added over a period ofabout 2 hours at 115° C. The remainder of the methyl isobutyl ketone wasadded as a rinse. The reaction was held at 115° C. for 4 hours. Thereaction mixture was then cooled to 100° C., and the ethyl alcohol wasadded. The reaction mixture was then heated to 105° C. and held for 2hours and then stripped to a reaction temperature of 125° C. to removethe ethyl alcohol. A total of 1054.8 grams of solvent was removed. Thereaction mixture was cooled to room temperature and 513 grams of methylisobutyl ketone was added to the reaction mixture to adjust solids toabout 70 percent. The reaction mixture had a solids content of 69.9percent and an acid value of 188.

EXAMPLE E

A polyacid half-ester of 1,4-cyclohexanedimethanol with a mixture ofmethylhexahydrophthalic anhydride and hexahydrophthalic anhydride(1,4-CHDM/HHPA/MHHPA) was prepared from the following mixture ofingredients:

    ______________________________________                                                             Parts by Weight                                          Ingredients          (in grams)                                               ______________________________________                                        1,4-cyclohexanedimethanol                                                                          216.0                                                    hexahydrophthalic anhydride                                                                        143.2                                                    methylhexahydrophthalic anhydride                                                                  338.1                                                    butyl CELLOSOLVE (trademark                                                                        232.4                                                    Union Carbide)                                                                deionized water      211.4                                                    dimethylethanolamine 253.6                                                    ______________________________________                                    

The cyclohexanedimethanol and hexahydrophthalic anhydride were chargedto a reaction vessel and heated under nitrogen atmosphere to 115° C. Themethylhexahydrophthalic anhydride was added over a period of about twohours at 115° C. The reaction mixture was then held at 115° C. for fourhours. The reaction mixture was then cooled to 100° C. followed by theaddition of butyl CELLOSOLVE and a mixture of deionized water anddimethylethanolamine and then cooled to room temperature. The reactionmixture had a solids content of 51.5% and an acid value of 113.1.

EXAMPLE F

A polyacid half-ester of1-(3-hydroxy-2,2-dimethylpropyl)-3-hydroxy-2,2-dimethyl propionate(Ester Diol 204) and adipic acid (ED-204/Adipic Acid) was prepared fromthe following mixture of ingredients:

    ______________________________________                                                          Parts by Weight                                             Ingredients       (in grams)                                                  ______________________________________                                        Ester Diol 204    614.4                                                       Adipic acid       876.0                                                       Hypophosphorous acid                                                                             0.3                                                        Xylene            263.0                                                       Butyl CELLOSOLVE  691.1                                                       Deionized water   848.2                                                       Dimethylethanolamine                                                                            534.0                                                       ______________________________________                                    

The Ester Diol 204, adipic acid, hypophosphorous acid and xylene werecharged to a reaction vessel and fitted with a reflux condenser and aDean-Stark trap filled with xylene. The reaction mixture was heatedunder nitrogen to reflux and held at reflux while removing water untilan acid value of 204 was reached. The reaction mixture was then cooledto 125° C. and stripped to 100% total solids. The reaction mixture wasthen cooled to 90° C. followed by the addition of butyl cellosolve and amixture of deionized water and dimethylethanolamine and cooled to roomtemperature. The reaction mixture had a solids content of 40.4% and anacid value of 99.8.

EXAMPLE G

A polyacid half-ester pentaerythritol and methylhexahydrophthalicanhydride (PE/MHHPA) was prepared from the following mixture ofingredients:

    ______________________________________                                                             Parts by Weight                                          Ingredients          (in grams)                                               ______________________________________                                        pentaerythritol      1089.3                                                   methylhexahydrophthalic anhydride                                                                  5274.4                                                   butyl acetate        1257.8                                                   n-propanol           1736.9                                                   ______________________________________                                    

The pentaerythritol and butyl acetate were charged to a reaction vesseland heated under nitrogen atmosphere to 100° C. Themethylhexahydrophthalic anhydride was added over a period of about twohours at 100° C. The reaction mixture was then held at 115° C. for fourhours. The reaction mixture was then cooled to 100° C. followed by theaddition of n-propanol. The reaction mixture was then heated to 105° C.The reaction mixture was then cooled to room temperature. The reactionmixture had a solids content of 72.3% and an acid value of 189.8.

EXAMPLE II

An aqueous coating composition for evaluation of each of the oligomericesters was prepared in the following manner:

An aluminum pigment paste was prepared by mixing together the following:

    ______________________________________                                        Amount (g)                                                                             Material                                                             ______________________________________                                        26.3     ethylene glycol monohexyl ether                                       4.0     propylene glycol monopropyl ether                                    14.3     poly(propylene glycol) of molecular weight 425                        3.6     U.V. absorber (TINUVIN 130 from Ciba Geigy                                    Corp.)                                                                0.7     phosphatized epoxy*                                                  34.8     treated aluminum pigment (HYDROLUX 400                                        from Obron Corp.)                                                    53.3     CYMEL 385 (aminoplast resin from                                              American Cyanamid)                                                   ______________________________________                                         *The phosphatized polyepoxide was prepared in the following manner: A         mixture of 266.7 g of 85% phosphoric acid and 344.4 g of ethylene glycol      nbutyl ether was heated to 110° C. under nitrogen atmosphere. A        solution of 1105.0 g of EPON 828 (a diglycidyl ether of bisphenol A which     is commercially available from Shell Chemical Company), 545.2 g of            ethylene glycol nbutyl ether, 38.2 g of xylene and 1.10 g of                  ethyltriphenylphosphonium iodide (commercially available from Morton Thio     ol Company) was added to the phosphoric acid solution over a two hour         period. Then 52.6 g of ethylene glycol nbutyl ether was used to rinse the     addition funnel and added to the solution. The solution was held at           110° C. for an additional 2 hours and 44.0 g of additional ethylen     glycol nbutyl ether were added. The final product had a total solids          content of 61.5%, a Gardner Holdt viscosity of X, and a milliequivalents      of acid per grams of 1.650.                                              

The aforesaid ingredients were stirred for 15 minutes and allowed tostand for one hour.

The latex prepared in Example I, above, was neutralized to a pH of about8.8 as follows:

    ______________________________________                                        Amount (g) Material                                                           ______________________________________                                        88.7       latex of Example I                                                 1.2        50 percent solution of dimethylethanolamine                                   in deionized water                                                 7.2        aliphatic hydrocarbons (SHELL SOL 71)                              8.4        diethylene glycol monobutyl ether                                  41.3       deionized water                                                    8.4        propylene glycol monopropyl ether                                  ______________________________________                                    

The coating composition was prepared by combining together the aluminumpigment paste and neutralized latex.

Each of the oligomeric esters described in Examples A through G, abovewas evaluated in the aforedescribed coating composition. Each of theesters was added to the coating composition in an amount of 20 weightpercent based on resin solids with agitation. The pH was then adjustedto 8.8 by adding an appropriate amount of a 50 percent solution ofdimethylethanolamine in deionized water.

Each of the coating compositions had spray solids of about 32 percentand a spray viscosity (number 4 Ford cup) of 24 to 26 seconds.

The pigmented coating composition was evaluated for use as a basecoatingcomposition over cold rolled steel treated with BONDERITE 40,commercially available from ACT and electrocoated with cationicallyelectrodepositable primer commercially available from PPG Industries,Inc. as ED-11.

The ED-11 coated panels, which were 12"×18" (30.5 cm×45.7 cm) in size,were primed with a commercially available PPG European primer surfacercoded E 730G305. This primer surfacer was cured for 25 minutes at 329°F. (165° C.).

The basecoat was spray applied to the primed substrate at 60% relativehumidity; 75° F.-80° F. (24° C.-27° C.) using a Devilbiss spray gunusing an AV1915 FX needle and 797 air cap with a 250 cc/minute fluiddelivery rate and flash baked 5 minutes at 200° F. (93° C.). Thebasecoat film thickness ranged between 0.5-1.0 mils (12.7-25.4 microns).

The basecoated panels were cured for 30 minutes at 250° F. (121° C.).The cured film was evaluated by visual inspection for areas of lightnessand darkness, also known as striping or mottling. In addition,brightness of face and face/flop properties were also visuallyinspected.

The results appear in the table below:

    ______________________________________                                                                Visual Mottling                                       Oligomeric Ester Additive                                                                             Rating                                                ______________________________________                                        none                    -                                                     ED-204/MHHPA of Example A                                                                             ++                                                    Di-TMP/MHHPA of Example B                                                                             +                                                     TMP/MHHPA of Example C  +                                                     NPG/MHHPA of Example D  +                                                     1,4-CHDM/HHPA/MHHPA of Example E                                                                      ++                                                    ED-204/Adipic Acid of Example F                                                                       ++                                                    PE/MHHPA of Example G   +                                                     ______________________________________                                         ++: complete elimination of mottling and striping                             excellent face/flop                                                           very bright face, very chromatic                                              +: very slight mottling and striping                                          improved face/flop                                                            bright face                                                                   -: severe mottling and striping                                               average face/flop                                                             average face brightness                                                  

We claim:
 1. An aqueous based coating composition containing a polymericfilm forming resin; characterized in that the aqueous based coatingcomposition contains as an additive from about 1 to about 40 percent byweight based on resin solids of the composition of an oligomeric esterhaving an acid value of from about 100 to about 500 of the structure##STR6## where X is the residue of a polyol containing from 1 to 50carbon atoms per hydroxyl group after reaction with an acid or ananhydride, R is an organic moiety from the acid or anhydride, and A hasan average value of 2 or greater.
 2. The coating composition of claim 1which contains from about 1 to about 25 percent by weight of theoligomeric ester.
 3. The coating composition of claim 1 in which thepolyol is selected from the group consisting of1-(3-hydroxy-2,2-dimethyl propyl)-3-hydroxy-2,2-dimethylpropionate,1,4-cyclohexanedimethanol, trimethylolpropane, di-trimethylolpropane,neopentylglycol, 1,4-butanediol, 1,6-hexanediol, and pentaerythritol. 4.The coating composition of claim 1 in which R is from an aromatic orcycloaliphatic anhydride containing from 2 to 30 carbon atoms.
 5. Thecoating composition of claim 4 in which R is from a polycarboxylic acidanhydride selected from the group consisting of alkyl substitutedphthalic anhydride, hexahydrophthalic anhydride, and alkyl substitutedhexahydrophthalic anhydride.
 6. The coating composition of claim 1wherein the oligomeric ester is the reaction product of1-(3-hydroxy-2,2-dimethylpropyl)-3-hydroxy-2,2-dimethylpropionate andmethyl hexahydrophthalic anhydride.
 7. The coating composition of claim1 wherein the oligomeric ester is the reaction product ofcyclohexanedimethanol 1,4 and a mixture of methylhexahydrophthalicanhydride and hexahydrophthalic anhydride.
 8. The coating composition ofclaim 1 wherein the oligomeric ester is the reaction product of1-(3-hydroxy-2,2-dimethylpropyl)-3-hydroxy-2,2-dimethyl propionate andadipic acid.
 9. The coating composition of claim 1 in which thefilm-forming resin contains functional groups which are capable ofreacting with a crosslinking agent.
 10. The coating composition of claim9 which contains a crosslinking agent.
 11. The coating composition ofclaim 10 wherein the crosslinking agent is an aminoplast resin.
 12. Thecoating composition of claim 1 wherein the volatile organic content isless than 3.5 pounds per gallon.
 13. The coating composition of claim 1wherein the film-forming resin is a latex which comprises polymericmicroparticles prepared by forming a mixture in aqueous medium of avinyl monomer or mixture of vinyl monomers with greater than 30 percentby weight of a substantially hydrophobic polymer; the percent by weightbeing based on weight of vinyl monomer(s) and hydrophobic polymer; saidpolymer being essentially free of repeating acrylic or vinyl units inthe polymer backbone and having a molecular weight of greater than 300,and particularizing the mixture into microparticles by high stresstechniques followed by polymerizing the vinyl monomer(s) to form themicroparticles which are stably dispersed in the aqueous medium.
 14. Thecoating composition of claim 13 wherein the hydrophobic polymer is apolyester or polyurethane.
 15. The coating composition of claim 13wherein the microparticles are crosslinked.
 16. The coating compositionof claim 13 which contains a crosslinking agent.